1. Field of the Invention
The present invention relates to wiring techniques for semiconductor devices, and more particularly to a wiring structure for highly integrated semiconductor devices having a passivation film disposed on a copper thin film for the wiring and made of an intermetallic compound formed by a reaction with copper to improve the conductivity and reliability of the copper wiring.
2. Description of the Prior Art
As semiconductor devices recently developed require a more rapid operation speed thereof because their integration degree has been abruptly increased. Due to such a requirement, a large amount of current flows through wirings of the semiconductor devices.
The increased integration degree of such semiconductor devices involves inevitably a decrease in line width of wirings, thereby increasing the density of current flowing through the wirings.
To this end, inexpensive aluminum exhibiting a good ohmic contact characteristic and a high conductivity have been used to make wirings or via lines. Where pure aluminum is used to make wirings, subsequent processes to be carried out after the formation of pure aluminum wirings should be those of low temperature. Moreover, such pure aluminum wirings involve a junction spike or an electromigration.
Due to such disadvantage of pure aluminum, aluminum alloys added with Si, Cu, Ni or Cr have been mainly used to make wirings of semiconductor devices, in place of pure aluminum.
Considering that the high integration of semiconductor devices are progressing continuously, such aluminum alloy wirings also have many disadvantages in terms of resistance and reliability, as in the pure aluminum wirings. For solving this problem, there has been proposed use of copper exhibiting a better conductivity than that of aluminum alloy by two times or greater.
In case of copper wirings, diffusion of copper to silicon substrates may occur. Moreover, the copper may be easily oxidized or etched by air existing in the atmosphere or process atmospheres used in process steps. Copper also has a poor adhesion to insulating layers containing oxygen.
Many researches and developments have been made in order to overcome the above-mentioned problems. One is disclosed in U.S. Pat. No. 4,742,014 relating to fabrication of copper wiring structures for semiconductor devices. Referring to FIGS. 1A to 1C, this method will be described.
As shown in FIG. 1A, a substrate 1 made of single crystalline silicon is formed with a diffusion region 2 near its surface. An insulating layer 3 is then coated over the surface of silicon substrate 1.
In order to expose the diffusion region 2, the insulating layer 3 is then partially removed at its portion disposed over the diffusion region 2, thereby forming a contact hole 4.
Over the entire exposed surface of the resulting structure including the surface of insulating film 3 and the surface of exposed diffusion region 2, a molybdenum thin film 5 is deposited using a sputtering method or a chemical vapor deposition method, as shown in FIG. 1B.
Thereafter, a copper thin film 6 is coated over the molybdenum thin film 5. The copper thin film 6 and molybdenum thin film 5 are sequentially patterned in this order.
As shown in FIG. 1C, a tungsten thin film 7 is then selectively deposited over only the patterned copper thin film 6 using a selective tungsten deposition method, thereby passivating the copper thin film 6.
Accordingly, the copper thin film 6 is used as a conduction layer whereas the molybdenum thin film 5 is used as a lower diffusion barrier layer for the copper thin film 6. On the other hand, the tungsten thin film 7 deposited on the upper and side surfaces of copper thin film 6 serves as an upper and side diffusion barrier layer for the copper thin film 6. Thus, the copper thin film 6 is encapsulated by the molybdenum thin film 5 of high melting point metal and the tungsten thin film 7.
Where the insulating layer 3 is an oxygen-containing insulating layer, the tungsten thin film 7 serves to prevent the copper thin film 6 from being oxidized by the oxygen of the insulating-layer 3. In this case, the tungsten thin film 7 also prevents the copper atoms of the copper thin film 6 from being diffused in the insulating film 3.
Meanwhile, the molybdenum thin film 5 serves to not only reduce the ohmic contact resistance between the copper thin film 6 and the diffusion region 2, but also to prevent the copper atoms of the copper thin film 6 from being diffused in the insulating film 3.
However, this method has a problem that a complicated fabrication is required to form the lower diffusion barrier layer, the conduction layer, namely, copper layer, and the upper and side diffusion barrier layer.
Furthermore, this method is difficult to prevent the copper thin film from being formed at the surface thereof with a natural oxide film in the atmosphere or a tungsten depositing atmosphere because it uses the selective tungsten deposition technique that is not a completely developed technique. For this reason, the method has a great difficulty to selectively form the tungsten thin film on the copper thin film.
As a means of settling the formation of the natural oxide film on the copper thin film, there has been proposed a method for removing the natural oxide film using a sputtering etch process in an in-situ fashion, prior to the deposition of the tungsten thin film, in the same CVD equipment as that used for the deposition of the tungsten thin film. However, this method has a difficulty to completely remove the residue of natural oxide film left on the side surfaces of copper thin film because it uses the sputtering etch technique having the anisotropic etch characteristic. As a result, it is difficult to form a satisfactory selective tungsten thin film due to the natural oxide film left on the side surfaces of copper thin film.
In this regard, the above-mentioned conventional techniques for passivating wirings of copper thin films using the selective tungsten deposition method need more researches in order to achieve the application of copper thin film wirings to highly integrated semiconductor devices.
Another conventional method for fabricating copper wirings of semiconductor devices is disclosed in U.S. Pat. No. 5,130,274. By referring to FIGS. 2A to 2C, this method will be described.
As shown in FIG. 2A, a copper thin film 11 is coated over the entire surface of a single crystalline silicon substrate (not shown) and then patterned to form a first wiring.
An insulating film 13 comprised of, for example, an oxide film is then deposited over the entire surface of the resulting structure including the surface of the patterned copper thin film 11 and the surface of the silicon substrate exposed after the patterning.
Thereafter, the insulating film 13 is partially removed at its portion corresponding to a region where the copper thin film 11 will be in contact with a copper thin film (not shown) for a second wiring to be subsequently formed, thereby forming a via hole 14.
A thin film 15 made of copper alloy added with aluminum or chromium is then deposited over the entire surface of the resulting structure including the surface of insulating film 13 and the surface of copper thin film 11 exposed through the via hole 14, by using the sputtering method or CVD method.
Subsequently, the copper alloy thin film 15 is etched back so that it is left only in the via hole 14, thereby forming a plug, as shown in FIG. 2B.
The plug comprised of the remaining copper alloy thin film 15 is then annealed in an oxygen atmosphere. During the annealing of the plug, the aluminum or chromium atoms of the plug migrates to the surface of plug and reacts with the oxygen of the insulating film 13 at the interface between the plug and the insulating film 13, thereby forming an oxide film 17 of Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3, as shown in FIG. 2C.
After completing the annealing, the plug buried in the via hole 14 provides a pure copper wiring 16 passivated at its upper and side surfaces by the oxide film 17.
However, this method has a problem that the oxide film provides lower copper diffusion barrier effect than nitride films. Since the aluminum or chromium atoms contained in the copper alloy reacts with oxygen supplied at the surface of the insulating film to form the oxide film in this method, the volume of the pure copper wiring is reduced as the thickness of the oxide film increases. This results in an undesirable increase in the resistance of the pure copper wiring.